The present invention is directed to polycarbonate polymers which are resistant to weathering. More particularly, the present invention is directed to polycarbonate polymers comprising oxanilide structural units, methods for producing them, and articles made therefrom.
Polycarbonates are a widely employed class of polymers, in part because of their excellent physical properties including high impact strength. However, the resistance of polycarbonates to change upon exposure to weather phenomena (generally termed xe2x80x9cweatherabilityxe2x80x9d) is not ideal. In particular, long term color stability is a problem as polycarbonates are subject to yellowing, which detracts from the transparency and attractiveness of the polymers. Yellowing of polycarbonates is caused largely by the action of ultraviolet radiation, which is why such yellowing is frequently referred to as xe2x80x9cphotoyellowingxe2x80x9d. Numerous means for suppressing photoyellowing and other detrimental phenomena associated with weathering have been employed and proposed. Many of these involve incorporation in polycarbonate compositions of ultraviolet absorbing compounds (UVA""s). For the most part, UVA""s are low molecular weight compounds and they must be employed at relatively low levels, typically up to no more than about 1% by weight, to avoid degradation of the physical properties of the polymer such as impact strength and high temperature properties as reflected in heat distortion temperature. A long-felt need exists for polycarbonate compositions with effective resistance to weathering which do not require the addition of low molecular weight UVA""s.
An example of a typical UVA is an oxanilide-comprising compound available under the trade name SANDUVOR VSU from Clariant Corporation and having the formula (I): 
Commonly assigned U.S. Pat. No. 5,847,196 discloses a synthesis of mono- and difunctional oxanilides containing aromatic hydroxy functionality. However, the oxanilides made by the method taught therein are themselves highly colored. Also, there is no teaching therein of polycarbonates which might be derived from such oxanilides. There is a need for a method for preparing oxanilides containing aromatic hydroxy functionality which produces oxanilides with little or no color, which can be used to prepare essentially colorless, transparent polycarbonates.
After diligent experimentation the present inventors have discovered polycarbonates comprising oxanilide structural units which are essentially colorless and which exhibit excellent weatherability. Thus, in one embodiment the present invention is a polycarbonate comprising structural units derived from at least one dihydric phenol, a carbonate precursor, and at least one member selected from the group consisting of an oxanilide-comprising compound, an oxamate-comprising compound, and mixtures thereof. In another embodiment the present invention is a method for making a polycarbonate comprising structural units derived from at least one dihydric phenol and at least one member selected from the group consisting of an oxanilide-comprising compound, an oxamate-comprising compound, and mixtures thereof, which comprises the step of contacting under reactive conditions at least one dihydric phenol, a carbonate precursor, and at least one member selected from the group consisting of an oxanilide-comprising compound, an oxamate-comprising compound, and mixtures thereof. In still another embodiment the present invention is an article comprising a polycarbonate comprising oxanilide structural units. In still another embodiment the present invention is a method for making a symmetrical or unsymmetrical oxanilide-comprising dihydric phenol which comprises the step of contacting at least one aromatic amine bearing a hydroxy group with a diaryl oxalate or a dialkyl oxalate bearing alkyl groups each substituted with at least one electron-withdrawing group. In still another embodiment the present invention is a method for making an oxanilide-comprising monohydric phenol which comprises the step of contacting an aromatic amine bearing a hydroxy group with a diaryl oxalate or a dialkyl oxalate bearing alkyl groups each substituted with at least one electron-withdrawing group. Various other features, aspects, and advantages of the present invention will become more apparent with reference to the following description and appended claims.